The present invention relates to a control method and device for a four wheel drive power transmission system for a vehicle adapted for four wheel drive operation, and more particularly relates to such a control method and device for such a four wheel drive power transmission system, for a vehicle such as an automobile, which includes a torque transfer device for limiting the differential action of a differential device which is provided for distributing power between the front wheels of the vehicle and the rear wheels of the vehicle.
The present invention has been described in Japanese Patent Application Ser. No. Showa 61-061799 (1986), filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Application and the claims and the drawings thereof; a copy is appended to the present application.
Nowadays a greatly increasing number of automotive vehicles are being constructed with four wheel drive transmission systems, because such four wheel drive operation, in which all four wheels of the vehicle are powered from its engine via its transmission, is very suitable for driving on poor or slippery road surfaces such as in mud or over bad ground, or upon roads covered with mud, snow, ice, or rain. In other words, four wheel drive operation provides a much higher degree of stability and drivability for the vehicle in conditions where the coefficient of friction between the wheels and the surface upon which the vehicle is riding is relatively low. Also, four wheel driver operation is beneficial for aiding with hill climbing characteristics and high speed stability characteristics. Therefore, the so called full time four wheel drive type of transmission, which remains always engaged to four wheel drive without any episodes of two wheel driving, is becoming more and more popular.
In such a four wheel drive transmission system for an automotive vehicle, it is usual to provide a center differential device for distributing rotational power between the front wheels of the vehicle and the rear wheels of the vehicle, as well as the per se conventional rear differential device that provides differential action between the two rear vehicle wheels and the also per se conventional front differential device that provides differential action between the two front vehicle wheels. Such a central or front--rear differential device is provided in order to provide a differential action between said front vehicle wheels (considered as a pair) and said rear vehicle wheels (also considered as a pair) when the vehicle is turning around a curve, in order to eliminate the possibility of the occurrence of the so called tight corner braking phenomenon created by the difference in the turning radiuses of the front wheels of the vehicle and the rear wheels thereof (and also for various other reasons). And such provision of such a central or front--rear differential device is effective for achieving this result. However, a problem that arises with such provision of such a central or front--rear differential device is that, if at any time even one only of the four vehicle wheels should break away from the road surface and should start to spin, then no drive power or at least severely reduced drive power is provided to the other three vehicle wheels. This type of problem is particularly likely to arise in the event that the road conditions are poor due to rain, snow, dust, dirt, or the like which deteriorate the coefficient of the vehicle wheels on the road surface, and thereby vehicle drivability can be severely reduced.
In order to counteract this effect, it has been practiced to provide a device to such a front--rear differential device which prevents or which impedes (i.e. partially prevents) said front--rear differential device from performing differential action, in a selective fashion. When such a center differential action inhibition means, which typically may be a hydraulic clutch or a hydraulic brake, is actuated, it causes the differential action provided by said front--rear differential device between the front vehicle wheels and the rear vehicle wheels to be prevented or at any rate impeded, and instead said front vehicle wheels, considered as a pair, are driven from the vehicle engine via the front differential device, and also said rear vehicle wheels, considered as a pair, are similarly independently driven from said vehicle engine, similarly via the rear differential device. Thereby, the problem outlined above, of loss of power to the other three vehicle wheels when one of the vehicle wheels starts to spin, is obviated. Such types of structure are disclosed, for example, in Japanese Patent Application Laying Open Publication Ser. No. 59-184025 (1984), Japanese Patent Application Laying Open Publication Ser. No. 59-151661 (1984), and Japanese Patent Application Laying Open Publication Ser. No. 59-206228 (1984), none of which is it intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law.
The following type of problem, however, can tend to arise in a vehicle incorporating such a front--rear differential device fitted with such a differential action inhibiting means. There has always been the possibility that said front--rear differential device could become completely locked up, so that it totally failed to provide any differential action at all between the front vehicle wheels considered as a pair and the rear vehicle wheels considered as a pair, as a result of the clutch or brake therein locking up securely, and in such a case there would be a possibility of all of the torque supplied by the engine of the vehicle being transmitted to the rear wheels of the vehicle only, of course via the rear differential device, or to the front wheels of the vehicle only, similarly of course via the front differential device. This problem has been recognized in the past, and in order to cope with its effects it has been necessary, and has been practiced, to construct the front differential device to have such a high torque transmission capacity that it can cope by itself with transmitting all of the torque supplied by the engine of the vehicle; and, similarly, it has been necessary and has been practiced to construct the rear differential device to have a similarly high torque transmission capacity, so that it also can cope by itself with transmitting all of the torque supplied by said engine of said vehicle. As a consequence, both the front differential device and also the rear differential device of the vehicle have been required to be made rather large, which not only has increased their cost and their weight, but also by increasing their size has reduced the ground clearance of the vehicle, for the same suspension construction thereof.